The Aurelia Curse (Dragon Rider #3)

"Ben and the Greenblooms must protect a mythical new creature rising from the ocean--a creature who can bring either light or darkness to whoever it first meets. Now not even baby dragons can keep Ben and the Greenblooms from heading to California where the vast, mythical Aurelia will appear. It's a race against time to protect it, for if the Aurelia is hurt, all fabulous creatures will vanish from Earth. In the end, though, it may take the arrival of the original silver dragon, Firedrake, to help save them all"--